This invention relates generally to communication systems in which an information signal is used to modulate a carrier signal, which is subsequently demodulated in a receiver. More particularly, the invention relates to techniques for acquiring such a carrier signal from a relatively noisy background, when the actual frequency of the carrier is not accurately known. The frequency of a transmitted carrier signal can vary from an expected or nominal value for a number of reasons. Crystal oscillators used in transmitters can vary significantly in frequency. In space communications, substantial frequency shifts can be caused by the Doppler effect, depending on the relative velocity between the transmitter and receiver.
Since the frequency of the received carrier signal may not be the frequency expected and may vary with time, it is necessary to provide means for initially acquiring the carrier signal, and generally then staying "locked on" to the carrier so that communications will continue to be received in spite of further frequency variations. Typically, the received signal is mixed or heterodyned with another locally generated signal to produce a signal of intermediate frequency for processing in the receiver. One well known type of signal acquisition technique is known as automatic frequency control (AFC). An AFC circuit typically comprises a frequency discriminator and a local oscillator. The discriminator produces a signal indicative of the error or deviation of the intermediate frequency from a desired value, and the discriminator output is used to control the local oscillator, in order to produce an intermediate frequency signal of the desired frequency, no matter what the variations in frequency of the incoming signal.
Unfortunately, however, the discriminator is highly sensitive to noise, since it functions essentially as an amplitude detector, and noise of all frequencies produces a discriminator response just as the desired carrier signal does. Moreover, as the frequency of the incoming signal becomes centered on the discriminator characteristic, the signal level obtained necessarily decreases relative to a full-scale level, and this decrease is effectively a loss in signal-to-noise ratio.
In general, acquisition of a carrier signal requires that a desired frequency band be automatically tuned so that its center frequency lies within a narrow range, which will be referred to herein as the acquisition window. The center frequency of the desired band may initially lie anywhere within a wider range, which represents the range of maximum frequency uncertainty. Ideally, the acquisition process must satisfy desired acquisition time requirements for a particular application, and must be able to achieve these goals in spite of the presence of noise and interference signals.
It is also highly desirable that the acquisition system should operate in such a manner that there is virtually no possibility that the desired signal will not be centered in the acquisition window. More particularly, there should also be virtually no possibility that the system will lock onto a signal that is not the desired carrier signal. Such false locking may occur, for example, in acquisition systems that utilize frequency sweeping. In such systems, a modulation sideband within the acquisition window may be incorrectly processed as the carrier.
It will be appreciated from the foregoing that there is a clear need for a signal acquisition system that is capable of acquiring a desired carrier signal centered somewhere within a range of frequency uncertainty, even when the carrier signal is obscured by relatively large noise and interference components. As will be explained further below, the present invention satisfies this need.